Function tcbCreateEventHandler

packages/compiler/src/typecheck/ops/events.ts:286–328

* Creates an arrow function to be used as handler function for event bindings. The handler * function has a single parameter `$event` and the bound event's handler `AST` represented as a * TypeScript expression as its body. * * When `eventType` is set to `Infer`, the `$event` parameter will not

(
  event: BoundEvent,
  tcb: Context,
  scope: Scope,
  eventType: EventParamType | string,
  assertionExpression?: TcbExpr,
)

 * bindings. Alternatively, an explicit type can be passed for the `$event` parameter.
 */
function tcbCreateEventHandler(
  event: BoundEvent,
  tcb: Context,
  scope: Scope,
  eventType: EventParamType | string,
  assertionExpression?: TcbExpr,
): TcbExpr {
  const handler = tcbEventHandlerExpression(event.handler, tcb, scope);
  const statements: TcbExpr[] = [];

  if (assertionExpression !== undefined) {
    statements.push(assertionExpression);
  }

  statements.push(handler);

  let eventParamType: string | undefined;
  if (eventType === EventParamType.Infer) {
    eventParamType = undefined;
  } else if (eventType === EventParamType.Any) {
    eventParamType = 'any';
  } else {
    eventParamType = eventType;
  }

  // Obtain all guards that have been applied to the scope and its parents, as they have to be
  // repeated within the handler function for their narrowing to be in effect within the handler.
  const guards = scope.guards();
  let body = `{\n${getStatementsBlock(statements)} }`;

  if (guards !== null) {
    // Wrap the body in an `if` statement containing all guards that have to be applied.
    body = `{ if (${guards.print()}) ${body} }`;
  }

  const eventParam = new TcbExpr(
    `${EVENT_PARAMETER}${eventParamType === undefined ? '' : ': ' + eventParamType}`,
  );
  eventParam.addExpressionIdentifier(ExpressionIdentifier.EVENT_PARAMETER);

  // Return an arrow function instead of a function expression to preserve the `this` context.
  return new TcbExpr(`(${eventParam.print()}): any => ${body}`);
}